1. Technical Field
The present invention relates to photovoltaic devices and fabrication methods, and more particularly to earth-abundant photovoltaic material having a titanium doped absorber layer.
2. Description of the Related Art
Cu2ZnSnSxSe4-x (CZTS) is a promising earth-abundant photovoltaic material for high-efficiency thin film solar cells. A power conversion efficiency (PCE) of 11.1% has been achieved by Teodor K. Todorov, et al. using a hydrazine-assisted solution approach (Adv. Energy Mater. 3, 34-38, 2013). Other vacuum-based and non-vacuum-based deposition techniques have also been successfully utilized to fabricate CZTS solar cells with PCE above 9%. However, it is very difficult to achieve an efficiency above 11%, and the PCE of CZTS solar cells is still far below the physical limit, known as the Shockley-Queisser (SQ) limit, of about 29% under terrestrial conditions.
One fundamental reason for the relatively low performance of CZTS solar cells is the large VOC deficit of CZTS devices. VOC deficit refers to the fact that VOC (open circuit voltage) is smaller than expected (e.g., a large VOC deficit corresponds to a smaller-than-expected VOC). VOC deficit is defined as Eg/q−VOC, where Eg/q is the band gap expressed in volts (q is the fundamental charge). Thus far, no effective method exists that solves the high VOC deficit issue.